This invention relates to a matrix display device for displaying information which includes optical sensing means enabling input of information as well, for example by way or a light pen. More particularly, the invention relates to a combined matrix display and light sensing device comprising an array of display elements, an array of light sensing elements which are responsive to illumination so as to provide an electrical charge indicative thereof, first and second sets of address conductors connected with the arrays of display and light sensing elements, the display elements and the light sensing elements each being connected with an address conductor of each set, a first drive circuit connected to the first set of address conductors for scanning the arrays, a second drive circuit for providing data signals which are supplied to the display elements via address conductors of the second set, a detection circuit for sensing the charge provided by the sensing elements and providing an output in accordance therewith and comprising charge sensitive amplifier circuits each having an output, a first input connected to an address conductor of the second set, a second input for receiving a read potential and a capacitor and switch connected in parallel between the output and the first input, and control means for opening and closing the switch of each charge sensitive amplifier circuit periodically and providing a read potential to the second input.
This kind of device may be used for example as an output/input interface for a computer system through which information can be displayed to a user and information can be entered by a user by writing with a light pen.
An example of such a device is described in EP-A-0491436. This device comprises a row and column array of liquid crystal display elements, each of which includes a switching element in the form of a thin film transistor (TFT), combined with a row and column array of light sensing elements which each comprise a photo-sensitive element, for example a photo-diode or photo-resistor, connected to a capacitor and a switching element, also comprising a TFT. The TFTs of the display elements and the sensing elements are connected to row and column address conductors. The display elements are driven in conventional fashion by a scan drive circuit which applies a selection signal to each row conductor in turn, and a data signal drive circuit which applies display data signals to the column conductors. The data signal drive circuit can be of a conventional kind used in matrix display devices comprising a sample and hold circuit operated by a shift register to perform serial to parallel conversion of a serial data signal input. A sampled data signal is supplied to a column of display elements via a column address conductor by a respective stage of the sample and hold circuit. It is common in such data signal drive circuits for the sampled data signals to be fed to the column address conductor via respective buffer amplifiers.
The display and sensing elements share the same row and column address conductors with an associated pair of display and sensing element conductors being connected to a respective column address conductor. A row selection signal provided by a scan drive circuit defines a row address period during which display information is written into a row of display elements and a read out is provided from the associated row of sensing elements. A charge is periodically applied to the capacitors of the sensing elements and this charge is changed in the intervening periods if the photosensitive element is illuminated. The charge state of the capacitors is monitored to provide an indication of whether or not the sensing elements have been illuminated by means of a detection circuit which includes for each column of sense elements a respective charge sensitive amplifier circuit connected to a column address conductor providing a read-out in accordance with the charge state of the sensing elements.
The use of charge sensitive amplifier circuits for providing a read-out from an array of light sensitive elements follows known practice in matrix light sensing devices, without a display function, for example as described in EP-A-0237365.